keyboard: Don't apply "Backspace" straight away

Before, when pressing "Backspace" in the editing dialogue, the
keybinding would be changed straight away, *behind* the dialogue, and
the dialogue would still be expecting a new shortcut.

Instead, we should make it behave like other shortcuts, which means
special handling empty shortcuts as valid ones.

https://bugzilla.gnome.org/show_bug.cgi?id=769314